The use of insecticides, fungicides or herbicides has been indispensable to the cultivation of field crops. When applying a pesticide, it is preferably applied in a large amount at a time from the viewpoint of the reduction of labor required for the application thereof, but such application suffers from various problems such that crops and human bodies are damaged by such a high concentration of the active ingredient, that the active ingredient is lost through washing away and that the duration of the efficacy thereof is correspondingly reduced.
In case of the crops which require the transplantation of the seedling such as those represented by paddy rice, they are damaged upon transplantation, for instance, by cutting of hair-roots and breakage of the seedlings. Moreover, the seedlings after the transplantation are in an unstable state till complete rooting thereof because of the great difference between the environmental conditions of nursery beds and final fields. For this reason, the application of a pesticide possessing a herbicidal action at this stage may bring about hypotrophy and withering of seedlings due to its phytotoxity.
If a pesticide such as a herbicide may be applied to seedlings when transplanting the seedlings, the labor required for farm working can effectively be reduced. In case of paddy rice, however, any herbicide can be applied thereto only after the complete rooting of the seedlings, i.e., after the lapse of about one week from the transplanting the seedlings, under the existing circumstances.
There have been known a variety of sustained release pesticides which are developed for eliminating such drawbacks and are so designed that active ingredients are gradually released from those applied to fields.
Japanese Patent Laid-Open Publication (JP-A) 286602/1990 discloses a granular controlled release pesticide which comprises mineral particles impregnated with a liquid active ingredient and hydrophobic fine particles which cover the surface of the mineral granules. However, the controlled release pesticide initiates the release of the active ingredient thereof immediately after the application thereof to fields and accordingly, it is not suitably applied to fields during the transplantation of seedlings.
Japanese Examined Patent Publication (JP-B) 5002/1989 discloses a sustained release pesticide which comprises a water-soluble or volatile active ingredient covered with a thermoplastic resin. In the controlled release pesticide, the active ingredient is released through the thermoplastic resin film. Therefore, this technique is suitable for water-soluble or volatile active ingredients, but it is difficult to apply the technique to hardly water soluble active ingredients. In general, most of the active ingredients used in herbicides are hardly soluble in water and if these active ingredients are covered with the films disclosed in JP-B 5002/1989, the resulting pesticide suffers from such problems that a desired herbicidal effect cannot be expected because of its extremely low release rate and that the active ingredient remains in the soil even after the harvesting of crops to thus cause contamination of the soil therewith.
JP-A 9304/1994 and JP-A 72805/1994 disclose timed-release, controlled release type pesticides, in which a part of the coating film is dissolved, after the application thereof to fields, to form openings through which the active ingredient is released. Moreover, JP-A 9303/1994 and JP-A 80514/1994 also disclose timed-release, controlled release type pesticides, in which the active ingredient is released through cracks formed, after the application thereof to fields, on a part of the coating film. In these timed-release, controlled release type pesticides, however, the timed-release characteristics are achieved by the use of a coating film having a double layered structure and the method for the production thereof requires complicated steps and the resulting product is expensive. Moreover, since the active ingredient is released through small openings or cracks formed on the film, the release rate thereof is show and thus it is difficult to apply these techniques to hardly water-soluble active ingredients.
The present invention intends to eliminate the foregoing drawbacks associated with the existing controlled release pesticides and accordingly, it is an object of the present invention to provide a coated granular pesticide which does not release any active ingredient immediately after the application thereof to fields and initiates the release of the active ingredient after the lapse of a desired period of time and which can complete the release of the active ingredient during the cultivation period, in particular, those effectively applied to hardly water-soluble active ingredients. It is another object of the present invention to provide a method for producing the same and a method for using the same.